This invention relates to a photosensitive resin composition and a photosensitive element. More particularly it relates to a photosensitive resin composition for forming a protective coating film having excellent characteristics which can be used, for example, for producing a printed wiring board and for metal precise processing, and to a photosensitive element comprising a layer of said composition and a support film supporting said layer.
It is well known that photosensitive elements, i.e., device comprising a support film and a layer of substantially dried photosensitive resin composition formed thereon are used as a photoresist for producing a printed wiring board. Further, it is also widely known that photosensitive resin compositions having excellent characteristics which can be used in a soldering mask, a resist for chemical plating, and the like are very useful.
Main objects of a soldering mask are to limit a soldering region at the time of soldering to prevent soldering bridge and the like; to prevent corrosion of a naked copper conductor, and to keep electric insulation between conductors. For these objects, thermosetting ink such as epoxy resin or the like, or photosetting ink has heretofore been screen-printed. However, in recent years, a wiring density of a printing board has been increased, and a soldering mask used for it has become difficult, with respect to precision, to form by a screen-printing method. With improvement in wiring density, electric insulation between conductors has come to be more severely required, and the thickness of a conductor-protecting film has come to be required to be at least about 20 .mu.m or more. When a screen-printing method is used, the average thickness of a resist which can be formed at a time is at most 30 .mu.m, and the thickness of the thinnest part of a resist formed on the projecting part of a conductor inevitably becomes 10 .mu.m or less. When printing is repeated two or three times, a thick film can be obtained, however, it is substantially very difficult with respect to printing precision and because of process complications. Therefore, the advent of a photosensitive element for forming a soldering mask is desired. Photosensitive elements in which the thickness of a layer of a photosensitive resin composition is 20 .mu.m or more are useful. Since the thickness of a conductor in most printed wiring is 18 .mu.m or more, photosensitive elements in which the thickness of a layer of a photosensitive resin 40 .mu.m or more are particularly useful.
In general, photosensitive elements for etching or electrolytic plating which are called photosensitive dry films and used for forming a conductor pattern of a printed wiring board do not have sufficient heat resistance, and hence cannot be used for forming a soldering mask.
Accordingly, there have been presented many proposals of a photosensitive resin composition for a photosensitive element good in heat resistance which can be used for forming a soldering mask (for example, Japanese patent application Kokai (Laid-Open) No. 56018/78 (U.S. patent application Ser. No. 735,979 filed Oct. 27, 1976), Japanese patent application Kokoku (Post-Exam Publn) No. 43,092/77 and Japanese patent application Kokoku (Post-Exam. Publn) No. 44,346/78 (U.S. patent application Ser. No. 782,378 filed Mar. 29, 1977)). The photosensitive resin compositions in these proposals are excellent in heat resistance which is one of the objects of the proposals, however when a thick protective film of 40 .mu.m or more in thickness is formed from the compositions, the film cracks within 5 cycles in a thermal shock test holding at 125.degree. C. then holding at -65.degree. C. repeatingly (MIL-STD-202E, method 107D, condition B). The thicker the film becomes, the more seriously the film cracks. This becomes a serious problem when the reliability of a printed wiring board for a long time should be taken into consideration.
Among soldering masks formed by a screen-printing method, there are those which are good in resistance to thermal shock. This is mainly because the thickness of soldering masks formed by a screen-printing method is 10 to 30 .mu.m. Another reason is that printing ink generally contains a large amount of a filler. It is well known that the presence of the filler contributes to improvement in the resistance to thermal shock. However, in the case of a photosensitive element, its layer of a photosensitive resin composition should substantially be dried before irradiation with actinic light, and should be film forming. This means that the layer of a photosensitive resin composition is required to contain a linear high-molecular compound to enhance its film forming ability. Therefore, it is difficult for the layer to contain a large amount of a filler in addition to a linear high-molecular compound, an actinic-lightsensitive compound and a sensitizer.
Further, the present inventors have done various researches to find that when a layer of a photosensitive resin composition for producing a photosensitive element is provided with about 10% by weight of a filler, the element gives a soldering mask improved in resistance to thermal shock but is deteriorated in soldering-heat-resistance which is one of the main characteristics of the mask, and hence is not practical.
As compounds used in photosetting ink for screen pringing, various urethane acrylate compounds or urethane methacrylate compounds (hereinafter referred to as "urethane (meth)acrylate compounds") have been proposed as those which are excellent in heat resistance. However, great difficulty is raised when these compounds are used not in photosetting ink but in the photosensitive element usually accompanied by imagewise exposure and development which is an object of this invention. A first difficulty is that many of the proposed urethane (meth)acrylate compounds are not miscible or are very poorly miscible with linear high-molecular compounds, particularly vinyl series copolymeric linear high-molecular compounds. A second difficulty is that many of the proposed urethane (meth)acrylate compounds are insoluble in noncombustible solvents used for the development of a photosensitive element, particularly in 1,1,1-trichloroethane series solvent which is generally most often used. Urethane (meth)acrylate compounds overcoming the above-mentioned difficulties and usable for providing photosensitive elements with excellent thermal shock and heat resistance have not yet been known.
One of conventional methods for improving the thermal shock is to use a compound sensitive to actinic light and having a larger molecular weight per photosensitive group. The molecular weight per photosensitive group is preferably 300 or more, more preferably 500 or more. But a cured film obtained from a photosensitive composition using a compound sensitive to actinic light and having a large molecular weight per photosensitive group is good in the thermal shock but is inferior in solvent resistance, heat resistance and the like properties. Therefore, the molecular weight per photosensitive group should properly be determined from the balance between the thermal shock and other properties such as solvent resistance, and the like.
On the other hand, U.S. Pat. No. 4,272,607 to Tsukada et al discloses a photosensitive resin composition useful as a solder resist including a linear polymer or copolymer having tetrahydrofurfuryl groups at side chains but is quite silent on the use of urethane diacrylate or dimethacrylate obtained from trimethylhexamethylene diisocyanate. U.S. Pat. No. 3,930,865 to Faust et al and U.S. Pat. No. 4,245,030 to Faust et al disclose photopolymerizable compositions which are still insufficient in heat resistance and adhesiveness of a coating film to a substrate after solder treatment, since these compositions are aimed at for producing printing plates, photoresists, etc, which do not require so high resistance to heat, or thermal shock as in the case of solder mask.
Therefore, it will be useful to discover special photosensitive compounds excellent in solvent resistance although the molecular weight per photosensitive group is large, or in contrast, to discover special photosensitive compounds having small molecular weight per photosensitive group and being excellent not only in solvent resistance, heat resistance, etc., but also in the thermal shock.